1. Field of the Invention
The present invention relates to an optical pickup, which enables optical disks having different standards regarding the thickness of a disk substrate, a recording density and so on to be recorded/reproduced as in the cases of a conventional low density optical disk, such as CD, CD-ROM and the like, and a high density optical disk, such as a digital video disk and the like (DVD, DVD-ROM).
2. Description of the Related Art
In order to reproduce information from two kinds of optical disks different in a substrate thickness and a recording density by the same optical system, there has been known a method using a bifocal condenser lens, which is described in pp. 27 to 29, "OPTICAL REVIEW" Vol. 1, No. 1 (1994).
A conventional optical pickup using a bifocal condenser lens will be described by referring to FIGS. 16A and 16B. FIGS. 16A and 16B each illustrates the configuration of an optical pickup, which is capable of performing reproducing from both of a conventional low density optical disk and a high density optical disk by using a bifocal condenser lens, and a light path. In FIG. 16A, reproducing of a conventional low density optical disk is shown. In FIG. 16B, reproducing of a high density optical disk is shown. The configurations of the optical systems are the same in FIGS. 16A and 16B, but light paths are different. A numeral 101 denotes a semiconductor laser, which has a wavelength shorter (636 nm to 650 nm) than the wavelength (780 nm) of a semiconductor laser used for the conventional low density optical disk. A numeral 51 denotes a conventional low density optical disk, which has a substrate thickness of 1.2 mm. A numeral 52 denotes a high density optical disk, which has a substrate thickness of 0.6 mm. In practice, recording/reproducing is performed by mounting any one of these optical disks on a spindle motor (not shown) for rotating a disk. A numeral 104 denotes a transparent substrate in which a concentric circular hologram element 105 is formed around an optical axis, and this transparent substrate is fixed to the same member as a condenser lens 106 and supported so as to be moved in focusing and tracking directions integrally with the condenser lens 106 by lens driving means (not shown). During recording/reproducing of data, in order to always form a very small spot in the recording surface of the optical disk 51 or 52, control is performed in a focusing direction so as to follow the face wobbling of a disk. Also, control is performed in a tracking direction in order that a spot may always follow a data track.
The operation of the foregoing conventional optical pickup will be described. A luminous flux radiated from the semiconductor laser 101 is raised in the direction of the Optical disk 51 or 52 by a half mirror 102, and converted into a parallel luminous flux by a collimator lens 103. The flux converted into the parallel luminous flux by the collimator lens 103 is made incident on the transparent substrate 104. A part of the luminous flux made incident on the transparent substrate 104 is diffracted by the hologram element 105, and the remaining parts are made incident on the condenser lens 106 without being diffracted. The condenser lens 106 is designed to fit the high density optical disk 52 having a disk substrate thickness of 0.6 mm, and thus the luminous flux passed through the transparent substrate 104 and moved straight ahead without being diffracted by the hologram element 105 can form a very small spot on the recording surface of the high density optical disk 52 having a substrate thickness of 0.6 mm. On the other hand, the hologram element 105 is designed in order that a 1st-order diffracted luminous flux may form a very small spot on the recording surface of the conventional low density optical disk 51 having a substrate thickness of 1.2 mm when this is converged by the condenser lens 106, and thus the 1-order luminous flux diffracted by the hologram element 105 can form a very small spot on the recording surface of the conventional low density optical disk having a substrate thickness of 1.2 mm. As described above, in the conventional optical pickup using the bifocal lens, recording/reproducing of the high density optical disk having a disk substrate thickness of 0.6 mm is performed by a luminous flux (0-order diffracted light), which is not diffracted by the hologram element 105. In this case, however, since the spot of a light diffracted by the hologram element 105 is deviated from a focus by a large amount, this is blurred and spread on the recording surface, and thus little influence is given to reproducing from the high density optical disk having a disk substrate thickness of 0.6 mm. Conversely, during reproducing from the conventional low density optical disk having a disk substrate thickness of 1.2 mm, this is performed by a 1st-order diffracted light of the hologram element 105. In this case, since the spot of a non-diffracted transmitted light is also deviated from a focus by a large amount, little influence is given to reproducing. A reflected light from the recording surface of the optical disk 51 or 52 is sent to the half mirror 102 through a path reverse to an outgoing path, converged on a photodetecting element 110 by a concave lens 109 after passing through the half mirror 102, and then a reproducing signal is detected.
FIGS. 17A and 17B each illustrates the configuration of another conventional example. In this example, a hologram element 105 is directly provided on a curved surface opposite a surface which faces the optical disk of a condenser lens 106. The other parts of the configuration and the operation are the same as in the example shown in FIGS. 16A and 16B.
Problems inherent in the foregoing conventional optical pickup are as follows.
The condenser lens 106 and the transparent substrate 104 in which the hologram element 105 is formed are made integral to each other and driven by the lens driving mechanism (not shown). Because of this integral formation of the condenser lens 106 and the transparent substrate 104, a movable part becomes thick and a weight is also increased. Consequently, it is also necessary to increase the size of the lens driving mechanism, and this in turn makes it difficult to miniaturize the optical pickup and reduce its weight.
Even if such an increase in the size of the movable part does not occur, it is difficult to accurately form a hologram element on a curved surface during manufacturing of a metallic mold for the condenser lens. Also, because of the existence of the hologram element, the number of lenses which can be formed by one metallic mold is smaller than a conventionally possible number, and thus manufacturing costs are increased.
Furthermore, during reproducing from the conventional low density optical disk 51, a 1st-order diffracted light diffracted by the hologram element 105 is used. During reproducing from the high density optical disk 52, a 0-order light not diffracted by the hologram element 105 is used. In practice, however, 1st-order or higher order diffracted lights are also produced. The existence of these lights which are not used reduces light utilization efficiency, and thus it is difficult to reduce the output of the semiconductor laser.